Technical Field
The present invention relates to the field of power amplifier technologies, and in particular, to an active bias circuit for a power amplifier and a mobile terminal including the active bias circuit.
Related Art
As well known, to implement high-speed data transmission, 3G communications systems including WCDMA/TDSCDMA and 4G communications systems including TD-LTE/LTE-FDD have much higher requirements on linearity of signals output by a power amplifier than communications modes such as GSM850/EDGE900/DCS 1800/PCS 1900. Therefore, the power amplifier is required to have a relatively high linear output power. It can be seen from FIG. 1 that duplexers and filters inevitably cause a signal loss, significantly increasing difficulty in maintaining high linearity of the power amplifier.
In the power amplifier, a bias circuit is responsible for providing a proper and stabilized bias state for a power-amplifier tube, and is important for maintaining linearity of the power amplifier. However, a bias circuit of a power amplifier in an existing GSM/EDGE/GPRS standard cannot be applicable to all currently mainstream 3G/4G mobile phones. A 3G/4G communications standard requires the power amplifier to have a higher power output. However, in a high power output state, a temperature of a chip of the power amplifier gradually rises (mainly a junction temperature of the power-amplifier tube rises). For the bias circuit, two conditions need to be met to keep the power-amplifier tube in a stabilized bias state as much as possible. First, the temperature of the chip is controllable, to avoid out of control or even burning of the chip resulting from a steady temperature increase and a current increase. Second, the power-amplifier tube is kept working in a relatively stabilized state by using a feedback mechanism, to suppress bias state shifting and linearity deterioration caused by temperature shifting.
Currently, some bias circuit structures meeting the foregoing conditions have been proposed in the industry and academia, but many bias circuit structures are insufficient to some extent in production and use, for example, increase process difficulty, cause a loss in power gains and efficiency, and increase a chip area and costs. For example, Chinese patent No. ZL 200780009732.5 discloses a dynamic bias control circuit for a power amplifier. However, the power amplifier is a design solution wasting a chip area. Correspondingly, a bias circuit structure introduced into the patent is relatively complex, and is inapplicable to chips of power amplifiers of 3G/4G mobile phones. For another example, Chinese patent No. ZL 01122022.8 discloses an active bias network circuit for a radio frequency amplifier. However, a bias circuit of a power amplifier in the patent has a complex structure, and a power amplifier module is required to include one independent bias circuit chip based on a complementary metal-oxide-semiconductor (CMOS) technology, increasing design implementation difficulty and costs of a chip of the radio frequency power amplifier.
It can be learned from the foregoing that bias circuits of current power amplifiers actually applicable to mobile communication still fail to meet requirements in practice.